1. Field of the Invention
This invention relates generally to implantable devices for interventional therapeutic treatment or vascular surgery, and more particularly concerns a coated superelastic stent formed from a stranded micro-cable with enhanced radiopacity.
2. Description of Related Art
The art and science of interventional therapy and surgery has continually progressed towards treatment of internal defects and diseases by use of ever smaller incisions or access through the vasculature or body openings in order to reduce the trauma to tissue surrounding the treatment site. One important aspect of such treatments involves the use of catheters to place therapeutic devices at a treatment site by access through the vasculature. Examples of such procedures include transluminal angioplasty, placement of stents to reinforce the walls of a blood vessel or the like and the use of vasoocclusive devices to treat defects in the vasculature. There is a constant drive by those practicing in the art to develop new and more capable systems for such applications. When coupled with developments in biological treatment capabilities, there is an expanding need for technologies that enhance the performance of interventional therapeutic devices and systems.
One specific field of interventional therapy that has been able to advantageously use recent developments in technology is the treatment of neurovascular defects. More specifically, as smaller and more capable structures and materials have been developed, treatment of vascular defects in the human brain which were previously untreatable or represented unacceptable risks via conventional surgery have become amenable to treatment. One type of nonsurgical therapy that has become advantageous for the treatment of defects in the neurovasculature has been the placement by way of a catheter of vasoocclusive devices in a damaged portion of a vein or artery.
Vasoocclusion devices are therapeutic devices that are placed within the vasculature of the human body, typically via a catheter, either to block the flow of blood through a vessel making up that portion of the vasculature through the formation of an embolus or to form such an embolus within an aneurysm stemming from the vessel. The vasoocclusive devices can take a variety of configurations, and are generally formed of one or more elements that are larger in the deployed configuration than when they are within the delivery catheter prior to placement. One widely used vasoocclusive device is a helical wire coil having a deployed configuration which may be dimensioned to engage the walls of the vessels. One anatomically shaped vasoocclusive device that forms itself into a shape of an anatomical cavity such as an aneurysm and is made of a preformed strand of flexible material that can be a nickel-titanium alloy is known from U.S. Pat. No. 5,645,558, which is specifically incorporated by reference herein.
The delivery of such vasoocclusive devices can be accomplished by a variety of means, including via a catheter in which the device is pushed through the catheter by a pusher to deploy the device. The vasoocclusive devices, which can have a primary shape of a coil of wire that is then formed into a more complex secondary shape, can be produced in such a way that they will pass through the lumen of a catheter in a linear shape and take on a complex shape as originally formed after being deployed into the area of interest, such as an aneurysm. A variety of detachment mechanisms to release the device from a pusher have been developed and are known in the art.
For treatment of areas of the small diameter vasculature such as a small artery or vein in the brain, for example, and for treatment of aneurysms and the like, micro-coils formed of very small diameter wire are used in order to restrict, reinforce, or to occlude such small diameter areas of the vasculature. A variety of materials have been suggested for use in such micro-coils, including nickel-titanium alloys, copper, stainless steel, platinum, tungsten, various plastics or the like, each of which offers certain benefits in various applications. Nickel-titanium alloys are particularly advantageous for the fabrication of such micro coils, in that they can have super-elastic or shape memory properties, and thus can be manufactured to easily fit into a linear portion of a catheter, but attain their originally formed, more complex shape when deployed. Although various materials are more or less kink resistant when nickel-titanium alloys are dimensioned into wire smaller than approximately 0.010 inches in diameter, they can have low yield strength and can kink more easily, thus severely limiting the applications for such finely drawn wire in the fabrication of vasoocclusive devices. As a further limitation to such applications, nickel-titanium alloys are also not radiopaque in small diameters, and a single nickel-titanium wire would need to be approximately 0.012 inches in diameter to be even slightly radiopaque. However, such a thickness of a single nickel-titanium wire would unfortunately also be relatively stiff and possibly traumatic to the placement site, particularly if used for treatment of delicate and already damaged areas of the small diameter vasculature such as an aneurysm in an artery or vein in the brain, for example.
One conventional guidewire for use in a catheter is known that is made of a high elasticity nickel-titanium alloy, and is useful for accessing peripheral or soft tissue targets. The distal tip of the guidewire is provided with a radiopaque flexible coil tip, and a radiopaque end cap is attached to the guidewire by a radiopaque ribbon. Such a construction is complex to manufacture, fragile and can potentially break off during use with undesirable results. A stretch resistant vasoocclusive coil is also known that can be made of a primary helically wound coil of platinum wire, with a stretch-resisting wire attached within the primary coil between two end caps. Unfortunately, such a construction is relatively difficult to fabricate and also fragile, allowing for the possibility of the fracture of the central radiopaque wire, the coil, the welds or some combination of them, and it can also potentially break off during use. Also, such a construction has a complex and nonlinear bending characteristic, dependent on the spacing of the coils and central wire and the radius of the bend of the coil.
Stents are typically implanted within a vessel in a contracted state and expanded when in place in the vessel in order to maintain patency of the vessel, and such stents are typically implanted by mounting the stent on a balloon portion of a balloon catheter, positioning the stent in a body lumen, and expanding the stent to an expanded state by inflating the balloon. The balloon is then deflated and removed, leaving the stent in place. However, the placement, inflation and deflation of a balloon catheter is a complicated procedure that involves additional risks beyond the implantation of the stent, so that it would be desirable to provide a stent that can be more simply placed in the site to be treated in a compressed state, and expanded to leave the stent in place.
Stents also commonly have a metallic structure to provide the strength required to function as a stent, but typically do not provide for the delivery of localized therapeutic pharmacological treatment of a vessel at the location being treated with the stent. Stents formed of polymeric materials capable of absorbing and releasing therapeutic agents may not provide adequate structural and mechanical requirements for a stent, especially when the polymeric materials are loaded with a drug, since drug loading of a polymeric material can significantly affect the structural and mechanical properties of the polymeric material. Since it is frequently desirable to be able to provide localized therapeutic pharmacological treatment of a vessel at the location being treated with the stent, it would be desirable to combine such polymeric materials with a stent structure to provide the stent with the capability of absorbing and delivering therapeutic drugs or other agents at a specific site in the vasculature to be treated.
Conventional forms of stents are known that have a covering or outer layers of collagen, that can be used for enhancing biocompatability and for drug delivery. One known tubular metal stent, for example, is combined with a covering sleeve of collagen in order to increase the biocompatibility of the stent upon implantation. The collagen sleeve may be collagen per se or a collagen carried on a support of Dacron or a similar material. Another three-layer type of vascular prosthesis has two outer layers formed of collagen, and a middle layer made from inert fibers such as synthetic fibers. Another tubular reinforcing structure for use as a cardiovascular graft is made of collagenous tissue with a reinforcing fibrous structure surrounding the lumen. One drug delivery collagen-impregnated synthetic vascular graft is also known, formed from a porous synthetic material having collagen such that the graft substrate cross-links the collagen in order to render the substrate blood-tight. A matrix is also provided in another biodegradable drug delivery vascular stent that is made from collagen or other connective proteins or natural materials that can be saturated with drugs. However, none of these types of stents provide for a coated, superelastic shape memory stent that can be delivered and released at the site in the vasculature to be treated in a compressed state, and expanded to leave the stent in place without the need for placement with a balloon catheter.
From the above, it can be seen that vasoocclusive devices and stents provide important improvements in the treatment of the vasculature. However, it would be desirable to provide a structural element that used to form a coated stent, that offers the advantages of a shape memory alloy such as a nickel-titanium alloy, and that incorporates radiopaque material in a stable configuration that is not subject to breaking during use of the device, so that the device can be visualized under fluoroscopy. The present invention meets these and other needs.
Significant advances have been made in the treatment of neurovascular defects without resolution to surgery. More specifically, micro catheters have been developed which allow the placement of vasoocclusive devices in an area of the vasculature which has been damaged. In presently used techniques, the vasoocclusive devices take the form of spiral wound wires that can take more complex three dimensional shapes as they are inserted into the area to be treated. By using materials that are highly flexible, or even super-elastic and relatively small in diameter, the wires can be installed in a micro-catheter in a relatively linear configuration and assume a more complex shape as it is forced from the distal end of the catheter.
In order to gain the advantages presently being realized with micro-catheter therapies and procedures to repair damage to the vasculature in the brain and other vessels, shape memory materials such as nickel-titanium alloys have been incorporated in vasoocclusive devices to be placed by the catheters. However, the range of diameters of wire and the configurations of the resulting geometry of both the coils and the devices developed which can be used have been limited by both the relatively small diameter of wire that must be used to avoid trauma and allow housing within the catheter prior to deployment, and the requirement for larger diameters to provide for radiopaque markers and mechanical robustness. In many cases this has resulted in primary wire characteristics in the coil that are unacceptably stiff, very delicate, or subject to kinking. The present invention obtains significant advantages over such prior art devices by providing a cable of multiple strands of an alloy adapted to be used in catheters, stents, vasoocclusive devices, guidewires and the like, thus providing a kink resistant, high strength material with highly desirable performance characteristics which can be altered by construction details to suit a variety of interventional therapeutic procedures.
More specifically, it has been found that single strands of small diameter nickel-titanium alloys, as well as other metal alloys, used to form vasoocclusive devices can be kinked if twisted and pulled as can occur during or after deployment from a catheter, especially if the doctor wishes to withdraw a partially deployed coil because it is somehow incorrect in size, shape or length to repair the damage to the vessel. Also, single wire coils are more likely to cause trauma to the area to be treated if the wire is of a sufficient diameter to provide adequate tensile strength. Furthermore, such small diameter wires of some of these materials such as nickel-titanium, stainless steel and the like, are not generally radiopaque with currently available equipment, necessitating the use of radiopaque markers attached to the device, with the resultant possible diminution of functionality and increased diameter.
The present invention solves these and other problems by providing, in its broadest aspect, a superelastic collagen coated stent formed from a micro-cable which includes at least one radiopaque strand to offer a continuous indication under fluoroscopy of the deployed configuration of the device incorporating the micro-cable. When combined with the benefits of a material such as nickel-titanium in the other strands of the micro-cable, numerous advantages are available from the use of this basic construction in interventional medicine. The shape of the superelastic collagen coated stent can contour to the shape of the anatomical cavity or portion of the vasculature, and the superelastic collagen coated stent would provide an adequate surface area of collagen for contact with a vessel wall to deliver drugs to the vessel wall.
Briefly, and in general terms, a presently preferred embodiment of the present invention provides for a superelastic collagen coated stent formed from a multi-stranded micro-cable made of a suitable material such as stainless steel or a nickel-titanium alloy, with the cable including at least one radiopaque strand, made of platinum, tungsten or gold, in order to serve as a marker during a procedure. The multi-stranded micro-cable can be configured into a stent to reinforce areas of the small diameter vasculature such as an artery or vein in the brain, for example. The superelastic collagen coated stent can be formed as a helical ribbon or tape, supported internally by a superelastic structure that can be compressed along the width and length of the stent structure, to be pushed by a pusher member through a microcatheter or cannula. When deployed in a helical configuration, with a ribbon cross-section, a closed helical pitch is achieved, providing a collagen tube in contact with the vessel wall. The stent is detachable from the pusher member. The superelastic collagen coated stent can be manufactured by producing the superelastic inner structure, compressing the structure, sliding it into a collagen tube, and allowing the superelastic inner structure to expand and flatten the tube into a ribbon. The collagen tube of the superelastic collagen coated stent is preferably loaded with a therapeutic agent or drug to reduce or prevent restenosis and thrombosis in the vessel being treated.
In one presently preferred embodiment, the invention accordingly provides for a superelastic collagen coated stent formed from a multi-stranded micro-cable having a plurality of flexible strands of a super elastic material, and at least one radiopaque strand. In one presently preferred embodiment, the multi-stranded micro-cable comprises a plurality of flexible strands of nickel-titanium alloy, the micro-cable having at least one central axially disposed radiopaque wire, such as platinum, tungsten or gold, for example, in order to provide a radiopaque marker during vascular procedures. In this preferred embodiment, the construction of the invention places the lowest tensile strength and highest flexibility member, the radiopaque marker strand, in a position in the cable which results in minimum stress on that member; at the same time, the superelastic material is in the outer strands, which have the dominant affect on performance parameters, thus enhancing the benefits of the material. Another benefit associated with the invention compared to prior art devices is that the multiple stranded cable configuration, in addition to providing a highly flexible and resilient structure, eliminates the necessity of a safety wire, since the failure of a single strand will not cause a severing of the cable. Also, the construction prevents stretching of the cable in the event of failure of a single strand, which is a significant benefit compared to constructions which have a coil around a central safety wire.
In a second presently preferred embodiment, the invention includes a superelastic collagen coated stent formed from a multi stranded cable constructed of multiple twisted strands of a suitable material such as a shape memory alloy or super elastic alloy of nickel-titanium, with one or more of the twisted strands consisting of a radiopaque material. The radiopaque strand may be one or more of the peripheral twisted strands and may also include one or more central strands of the cable. In a preferred aspect of the embodiment, the cable consists of six peripheral twisted strands and a central linear core strand, one or more of which can be of radiopaque material.
In a third aspect of the invention, the cable forming the superelastic collagen coated stent can be of linear strands that are arranged in a bundle and fastened or bound at intervals, or continuously, in order to maintain contact among the strands as the cable is bent. One or more of the strands may be radiopaque. This construction is adaptable to guidewires and other structures that must be pushable and/or torqueable, but still remain highly flexible and include radiopacity. Variations on this embodiment can include an outer sheath which consists of a solid or helically wound cover to provide enhanced torqueability and pushability. More specifically, the outer sheath can vary in thickness, stiffness of material or spring of the sheath members to provide desired variations in bending or stiffness of the cable. Such a construction is particularly adaptable to guidewires and the like, and can be varied in terms of the binding or outer layer to alter the torqueability of the cable, and the flexibility of the cable can be varied along its length by the number and sizes of the stranded members in the cable.
In a fourth aspect of the invention, one or more of the strands of the superelastic collagen coated stent can be of a therapeutic material used to enhance treatment of the site after placement of the device. In one presently preferred embodiment of the invention, the cable includes twisted strands of wire around the periphery of the cable, at least one of which is radiopaque. The core of the cable contains a therapeutic agent such as human growth hormone, genetic material, antigens or the like that are intended to become active after placement. Such a construction can be adapted to a variety of interventional therapeutic treatments. In one aspect of this embodiment, one of the strands can have multiple functions, such as providing both a therapeutic effect and also contributing to the structural integrity of the cable. By using copper in such a micro-cable, for instance, the copper can enhance the use of a device made from the cable as on intra-uterine device, with the copper also contributing to the radiopacity and structural integrity of the micro-cable. In the event that such an effect is desired, the therapeutic strand can be placed on the exterior of the cable to enhance contact with the site to be treated.